In this work, the authors consider the radiative transfer equation, in particular the approximation given by the Fokker-Planck equation. They introduce a weak formulation of the problem based on the splitting of functions into an even and odd part, and they prove its well-posedness. Moreover, they consider a Galerkin approximation based on spherical harmonics of an arbitrary order for the angular approximation and finite elements for the spatial approximation. Finally, they introduce an iterative method for the solution of the problem at hand and they prove its convergence.